The present invention concerns circuits used for communication systems and pertains specifically to a circuit that includes an amplifier and a bypass switch useful for low power devices that utilize microwave signals.
Modern digital wireless systems, such as those used in cellular phones and local area networks (LANs) utilize signals with a frequency typically in the range of 0.5 gigahertz (GHz) to 6.0 GHz.
For strong transmission signals, for example when there is a relatively short transmission distance, it is often advantageous to use a switch or series of switches to bypass a microwave amplifier in order to conserve power. The microwave amplifier, while important to achieve high dynamic range for weak transmission signals, may be unnecessary for strong transmission signals. Providing a bypass of the microwave amplifier allows the system gain to be lowered when the input signal is strong.
In the prior art, to provide for the bypass of a microwave amplifier, switch components are mounted on a printed circuit board. The switch components route the signal to the amplifier to perform the basic amplification function or to the output. Each switch component is, for example, a mechanical type switch, a solid state field-effect transistor (FET) switch or multiple diodes functioning as a switch. A typical topology of an amplifier with a bypass switch includes three switches. A switch is used to bypass the amplifier. A switch connected to the input of the amplifier and a switch connected to the output of the amplifier are used to isolate the amplifier when the bypass switch is turned on and the amplifier is being bypassed.
There are numerous disadvantages of the above-described prior art. Each switch and the amplifier require separate control and bias lines, complicating both layout and switch control. Numerous external components are required, adding both cost and space requirements. Since mechanical switches require 15V or more (handsets provide 2.7V), diodes must be used for switches, requiring significant current draw in the switch state. This current mitigates against the advantage of switching out the amplifier. And the series switches will reduce the performance of the amplifier.
An FET switch can be integrated into the same Integrated Circuit (IC) as the FET amplifier. For example, Ray Moroney, Kevin Harrington, Wayne Struble, Brian Khabbaz, Mike Murphy, A High Performance Switched-LNA IC for CDMA Handset Receiver Applications, 1998 IEEE Radio Frequency Integrated Circuits Symposium, p. 43-46, shows an integrated Gallium Arsenide (GaAs) FET solution where a switch function is integrated with an amplifier. This earlier implementation differs from the claimed by numerous points. The disclosed amplifier/switch (AS) uses only a series mode switch, limiting the gain of the amplifier. The disclosed circuit requires several blocking capacitors, increasing die size and cost.
Further, in the AS disclosed by Ray Moroney et al., when the switch is engaged, the input is connected to the output with only the on resistance of the switch in between. This state presents a good match. In order to present the same impedance in both the amplifier and the switch state, the amplifier state must also have a good match. To accomplish this match, the amplifier must be significantly fed-back. This feedback reduces the gain and increases the Noise Figure of the amplifier. This change in performance makes the AS incapable for the premiere slot in a receiver.